The present invention relates to a communication system and in particular, the present invention relates to a communication system for transmitting data packets via a plurality of virtual wireless communication links established between a base station and a plurality of operational mobile stations and having a variable transmission capacity.
Wireless communication networks for transmitting data and audio information are increasingly in use. Wireless communication networks are able to serve mobile stations as well as fixedly located end stations. A data transmission in such networks is often performed using multiplex techniques, where a plurality of communication links is transmitted via a single transmission channel. Such a transmission channel may, e.g. be a radio frequency band. Examples for wireless communication networks are mobile communication networks, e.g. networks according to the GSM-standard.
In communication networks employing multiplex techniques individual radio channels are partitioned into a plurality of time frames, itself each partitioned into a plurality of time slots. Each of these time slots can now be used for transmitting data for one communication link in that a sending station places data to be transmitted via one of the communication links onto particular time slots of the time frame. At a receiving station, it is known, which time slots are used for transmitting data intended for the receiving station and thus all data transmitted via the communication link can be recovered at the receiving station. For example, the receiving station combines the short data segments transmitted on the time slots to a continuous audio signal.
In general, different techniques may be employed to transmit data of a plurality of communication links via a common medium. It is a widely used method to employ a fixed assignment of time slots of a transmission frame to communication links. Such a fixed assignment, for example, is generally in use in GSM-systems.
If it is assumed that for example data of four communication links are to be transmitted via a common medium, each of the transmission frames of the medium could be divided into four time slots, and a first communication link could be established via the first time slot of each transmission frame, a second communication link could be established via the second time slot and analogously, the other communication links could be established via the third and fourth time slots.
Thus, the transmission medium is made available to each of the communication links at a fixed point in time. A fixed assignment of time slots to communication links thus allocates a predetermined and fixed capacity to each of the communication links. In general therefore the same channel capacity is made available to a communication link, which is used for the transmission of speech signals and accompanying speech pauses, as it is made available to a communication link used for the transmission of data, e.g. between two computers. However, in order to enable an efficient usage of a communication system it is desirable to adapt the capacity assigned to a communication link to the actual requirements of the amount of data to be transmitted.
A second generally used method for assigning of time slots to communication links enables an adaptation to the capacity requirements of a communication link. Here, a repetitive and fixed assignment of time slots of a communication channel to communication links it is not performed any longer, the assignment is performed dynamically corresponding to the amount of data to be transmitted.
One of the connection-oriented multiplex switching techniques is ATM (Asynchronous Transfer Mode). In ATM data belonging to a communication link are not necessarily transmitted periodically on a transmission medium and it becomes possible to allocate transmission bandwidth for subscribers as required, without encountering problems, if one of the subscribers requests to use a large bandwidth. ATM combines advantages of packet-switched networks and line-switched networks.
ATM covers layer 1 and large parts of layer 2 of the ISO-OSI-reference model. The OSI-reference model of the ISO (International Standardization Organization) describes a protocol layering for a communication between open systems, i.e. systems not in accordance with a standard, and distributed systems. Layer 1 of the OSI-reference model is the physical layer, layer 2 is the link layer.
In an ATM transmission so-called message cells of a predetermined length are transmitted as transmission units, using address information contained in a header. In general, ATM cells contain messages concerning the virtual connection, i.e. the communication link, as well as information concerning virtual communication channels, wherein a communication link may be divided into a plurality of virtual channels. Corresponding to the amounts of data individual channels may occupy more or less time slots.
A communication-oriented transmission of data via virtual communication links has been to date used particularly in communication networks with fixed lines, as for example in LANs (Local Area Networks).
It is, however, desirable to apply the advantages of a requirement-oriented data transmission to wireless communication networks. But in this endeavor it shows that data rates and bit error rates in communication networks with fixed lines and requirement-oriented transmission, e.g., a switching based on ATM, do not allow to readily use these techniques in wireless communication networks. For example, ATM for a data transmission in fixed networks provides a typical data and bit error rate of 155 Mbit/s and 10e-14, respectively. For providing the services of fixed networks with variable communication link capacity in mobile communication networks, an adaptation to a low channel capacity (25 Mbit/s) and a high bit error rate (10e-2) for radio channels (25 MHz channels in the 5 MHz range) is required.
Rules for an adaptation of connection-oriented switching techniques are, e.g. specified in the MAC (Medium Access Control) protocol. The MAC-protocol occupies a sub-layer of the layer 2 of the OSI-reference model.
The MAC-protocol consists of a number of messages, which are transmitted between mobile stations and a base station and determine the usage of the transmission medium. The MAC-protocol enables to establish a plurality of virtual communication links with communication-oriented transmission capacity using multiplex techniques. A MAC-protocol in general consists of different protocol data units for transmitting the above messages. Therefore, the efficiency and flexibility of a MAC-protocol is closely linked with the definition of different types of protocol data units (PDU).
A definition of protocol data units for a wireless requirement-oriented ATM communication is, e.g. known from xe2x80x9cDesign Principles for a MAC-protocol of an ATM air interfacexe2x80x9d, D. Petras, A. Krxc3xa4mling, A. Hettich, Mobile Telecommunications Summit, Granada, Spain, Nov. 27-29, 1996. In this document four types of protocol data units are described, transmitted in a transmission frame divided into an uplink and a downlink transmission portion. Here, in a uplink transmission portion data are transmitted from a mobile station to a base station and in a downlink transmission portion data are transmitted from a base station to a mobile station.
A Sig-PDU for uplink transmission contains a reservation message for a following transmission frame and further control information, e.g. for controlling access to time slots reserved for a transmission from an arbitrary mobile station.
A ATM-cell-PDU for downlink transmission contains an ATM-cell having a LLC (Logic Link Control) header with an overall length of 55 Byte.
A Sig-PDU, used for uplink transmission, contains, e.g., a capacity request or acknowledgments having a length of roughly 3 Byte.
A ATM-cell-PDU, used in uplink portions of the transmission frame contains an ATM-cell, a LLC (Logic Link Control) header and a capacity request message having an overall length of 57 Byte.
This partitioning of messages of the MAC (Medium Access Control)-protocol in four protocol data units, however, is disadvantageous, since the data transmission is not efficient. If n-ATM-cells of payload data are to be sequentially transmitted to a mobile station, control information is transmitted n-times, because it is part of each ATM-cell-PDU. It is sufficient, however, to transmit this control information a single time at the beginning of the n-ATM-cells. Furthermore, a UpATM-cell-PDU typically contains messages of varying importance and thus of varying sensitivity to errors. It is therefore difficult to provide efficient measures for error correction.
Further, a MAC-protocol with the name MASCARA is known from xe2x80x9cMASCARA, a MAC-protocol for wireless ATM,xe2x80x9d ACTS Mobile Communications, Granada, Spain, November 1996, pp. 647-651, F. Bauchot, S. Decrauzat, G. Marmigere, L. Merakos and N. Passas. The MASCARA-protocol defines the concept of a cell sequence, wherein a succession of ATM-cells is transmitted, which belong to the same virtual connection, and are transmitted as payload of a MAC-PDU (MPDU). Each MPDU comprises a MPDU-header, followed by a MPDU-body portion. Even though the MASCARA-protocol avoids various disadvantages of the above-mentioned MAC-protocol based on four types of PDUs, the MASCARA-protocol does not provide the needed flexibility to transmit short messages necessary for organizing data transmission.
It is therefore object of the invention, to provide a communication system enabling an efficient requirement-oriented data transmission.
This object is solved by a communication system for transmitting a plurality of payload data packets, comprising: a base station (B), a plurality of operable mobile stations (M1-Mn), communication links (C1-Cm) with variable transmission capacity, established between the base station (B) and the plurality of operable mobile stations (M1-Mn), each of the communication links occupying time slots of a radio channel for a data transmission, control data packets of a first type (S1), each of which being transmitted from the base station (B) via each of the virtual connections (C1-Cm) to each of the plurality of operable mobile stations (M1-Mn) and at least containing information on the partitioning of the radio channel into time slots, control data packets of a second type (S21-S24), each of which being transmitted via one of the virtual connections (C1-Cn), for controlling the virtual connection, and control data packets of a third type (S3), each of which being transmitted via one of the virtual connections (C1-Cm) and containing at least one payload data packet.
The object of the invention is further solved by a communication system for transmitting a plurality of payload data packets, comprising: a base station (B), a plurality of operable mobile stations (M1-Mn), communication links (C1-Cm) with variable transmission capacity, established between the base station (B) and the plurality of operable mobile stations (M1-Mn), each of the communication links occupying time slots of a radio channel for a data transmission, control data packets of a first type (S1), each of which being transmitted from the base station (B) via each of the virtual connections (C1-Cm) to each of the plurality of operable mobile stations (M1-Mn) and at least containing information on the partitioning of the radio channel into time slots, control data packets of a second type (S21-S24), each of which being transmitted via one of the virtual connections (C1-Cn), for controlling the virtual connection, control data packets of a third type (S3), each of which being transmitted via one of the virtual connections (C1-Cm) and containing at least one payload data packet, and forward error control signals transmitted in association with at least one of the control data packets, with a forward error control signal for a control data packet of the first type (S1) exhibiting a redundancy equal or larger than the redundancy of a forward error control signal for a control data packet of the second type (S21-S24), and a forward error control signal for a control data packet of the second type (S21-S24) exhibiting a redundancy equal or larger than the redundancy of a forward error control signal for a control data packet of the third type (S3).
Still further, the object of the invention is solved by a communication system for transmitting a plurality of payload data packets, comprising: a base station (B), a plurality of operable mobile stations (M1-Mn), communication links (C1-Cm), each divided into a plurality of channels, with variable transmission capacity, established between the base station (B) and the plurality of operable mobile stations (M1-Mn), each of the communication links occupying time slots of a radio channel for a data transmission, control data packets of a first type (Si), each of which being transmitted from the base station (B) via each of the virtual connections (C1-Cm) to each of the plurality of operable mobile stations (M1-Mn) and at least containing information on the partitioning of the radio channel into time slots, control data packets of a second type (S21-S24), each of which being transmitted via one of the virtual connections (C1-Cn), for controlling the virtual channels, and control data packets of a third type (S3), each of which being transmitted via one of the virtual connections (C1-Cm) and containing at least one payload data packet.
The invention advantageously provides several types of control data packets, which are transmitted between a base station and a plurality of operable mobile stations via virtual communication links having a variable transmission capacity.
Advantageously, the types of control data packets are designed to contain messages necessary for a requirement-oriented transmission of data, the messages being sorted according to their type and importance. Thus, error correcting means may be provided for the respective control data packets with varying importance, the error correcting means taking into account said importance. The introduction of error correcting means allows a very efficient forward error control already on the lowest, physical layer of a transmission medium.
Further, an adapted transmission capacity may advantageously be assigned for the transmission of control data packets according to the importance of their contents. Therefore, transmission capacity may be preferred assigned to control data packets of the first type and control data packets of the second type.
By choosing the contents of the control data packets, a high transmission efficiency with a small overhead is advantageously achieved. Further, the choice of the contents of the control data packets and their lengths allows a high flexibility and efficiency in transmitting information for controlling the virtual communication links.
Advantageously, control data packets of the second type may contain different kinds of control information for controlling a virtual communication link or a virtual communication channel, respectively. Thus, control data packets may be kept short for a higher flexibility and efficiency of the system. Time slots for the transmission of data via a virtual connection may be reserved or indicated by a time slot reservation signal in a control data packet of the second type. Further, signals for establishing a communication via the virtual link and/or signals for maintaining a communication link may be comprised therein. Further, a control data packet of the second type may contain an error control signal, which may be used for signaling the correctness or incorrectness of payload data packets transmitted via the virtual link.
In a further advantageous embodiment forward error control signals are transmitted at least for one type of control data packets. A forward error control signal for a control data packet of the first type advantageously exhibits a larger redundancy than a forward error control signal for a control data packet of the second type, and moreover, a forward error control signal for a control data packet of the second type exhibits a larger redundancy than a forward error control signal for a control data packet of the third type. Thus, the importance of the different kinds of control data packets may also be efficiently addressed via an error control.
Advantageously, the length of a control data packet of the third type may be an integer multiple of the length of a control data packet of the second type, thus further increasing transmission efficiency.
Further, with the definition of the kinds of control data packets and their structure, an analysis of control data packets may advantageously be performed directly, without awaiting an acknowledgment from a higher layer of a used communication model.
In a further advantageous embodiment of the invention, a data transmission in the communication system may be performed corresponding to the OSI-reference model. Further, control data packets may contain MAC-protocol data units and the data transmission may be performed via ATM.
Further advantageous embodiments become obvious with further dependent claims.